[clang] [Clang] Allow raw string literals in C as an extension (PR #88265)

[via cfe-commits]

cor3ntin wrote:

> C has Unicode string literals as well: https://godbolt.org/z/chdjYrK9v and so if we're allowing raw string literals, it makes sense to also allow raw unicode string literals IMO. I don't think we need to rename the flag though.

Yes, these things are completely orthogonal, it makes no sense to treat raw strings with an encoding prefix differently

https://github.com/llvm/llvm-project/pull/88265